1. Field of the Invention
The present invention relates to a secondary battery. More particularly, it relates to a secondary battery which can efficiently radiate the heat generated during the service, which is less likely to suffer from the electrolyte-leakage problems resulting from the internal-pressure increment, and which can stably carry out its functions.
2. Description of Related Art
In secondary batteries, such as lithium-ion batteries, the positive electrode and negative electrode usually employ a sheet-shaped electrode in order to take out electricity efficiently. The sheet-shaped electrodes are prepared by coating an active material thinly on a metallic sheet. The resulting sheet-shaped electrodes are faced with each other while interposing a separator therebetween. Moreover, in large-sized batteries (i.e., batteries of large capacity) used in electric vehicles, the positive electrode and negative electrode are required to have an enlarged area, but to be accommodated in a battery container compactly. In particular, the secondary batteries, referred to as cylinder-shaped batteries, employ electrode sheets which are made long and continuous. The long-and-continuous electrode sheets are rolled by using a mandrel for rolling, thereby preparing a cylindrically-formed electrode. The resulting cylindrically-formed electrode is accommodated in a cylinder-shaped container. Finally, the cylinder-shaped container is closed at the opposite ends by bonding in an air-tight manner.
When the secondary batteries are charged or discharged, they generate heat by the internal resistance. Especially, the large-sized secondary batteries produce a larger amount of heat, because a large electric current flows therein. Accordingly, it is necessary to suppress the internal-pressure increment resulting from the generation of heat, and to provide adequate pressure resistance for the battery container. In particular, the cylinder-shaped batteries produce a large amount of heat, and radiate the generated heat only at the outer peripheral portion and the opposite disk-shaped members limitedly. Accordingly, the cylinder-shaped batteries carry out the radiation of heat insufficiently at portions adjacent to the center of rolling. As a result, the internal pressure increases so that there occurs the warpage at the opposite disk-shaped members, and furthermore there arise the electrolyte-leakage problems at the bonding between the cylinder-shaped container and the opposite disk-shaped members.
Japanese Unexamined Patent Publication (KOKAI) No. 6-349,460 proposes one of the measures for inhibiting the large-sized batteries from increasing the temperature: for instance; a column-shaped battery is hollowed along the central axis to utilize the resulting hollow as a cooling cylinder. According to the technique, an inner cylinder is made from an electrically-conductive metal, and is used as an external terminal of a positive electrode or a negative electrode. Moreover, disk-shaped plates are disposed at the opposite ends of a container of the column-shaped battery; and at least one of the disk-shaped plates is bonded to the inner cylinder and an outer cylinder of the container by caulking, or is fastened thereto with screws, by way of an insulating gasket or packing in an air-tight manner.
In the application to electric vehicles, it is essential to connect the independent large-sized batteries in series in a quantity of about 100 in order to produce an output voltage of about 300 V. In addition, it is needed to gather them compactly. The large-sized batteries of the above-described conventional construction employ a battery container which works as the external terminal as well. Consequently, when the battery containers are brought into contact with each other, there occurs short-circuiting between the electrodes. Hence, when assembling the conventional large-sized batteries together, it is necessary to put an insulating cover on an outer periphery of the containers. As a result, there arises the problem in that the covers impair the heat radiation off from the conventional large-sized batteries. In addition, when the heat is generated at the central portion of the conventional large-sized batteries, the heat not only radiates off directly from the inner periphery of the inner cylinder, but also transfers to the opposite end plates. Therefore, it is needed to radiate the heat off from the surface of the opposite end plates. In the conventional technique disclosed in the publication, however, the heat is inhibited from transferring to the opposite end plates, because the end plates are bonded to the inner cylinder, or fastened thereto with screws, by way of the insulating member, or the like. Thus, there arises the problem in that the heat radiation cannot be carried out sufficiently. Moreover, the end plates are bonded to the outer and inner cylinders, for example, by caulking. Accordingly, the bonding strength is low between the end plates and the outer and inner cylinders. Thus, it is difficult to provide the air-tightness between the outer and inner cylinders and the end plates.